Solvent based acrylic conformal coatings have been historically the number one choice for moisture protection of printed circuit boards and they provide excellent humidity resistance.
They also dry quicker than nearly all the other conformal coating types, they are easy to use in production and are easily repaired.
However, acrylic conformal coatings have very little chemical resistance. So, they are poor at protecting circuit boards in harsh chemical environments.
This is where polyurethane conformal coatings are considered.
Using polyurethane conformal coatings to protect against chemical attack
Polyurethane (UR) coatings also provide good humidity & moisture protection compared to acrylic materials.
But, what is key to their selection as a protective conformal coating is that they also offer excellent chemical resistance to the circuit board.
The reason for this is that UR coatings cure rather than dry. That is they cross-link once applied to the circuit board by one of the many methods including solvent evaporation (initial stages), heat, UV, moisture and catalysed cure.
This cross-linking of the polymer chain provides the chemical resistance for which the UR coatings are generally selected.
This makes them excellent conformal coatings where the chemical attack is a potential hazard for the electronic circuit boards. This includes sectors such as aerospace, military and industrial sectors plus other diverse areas such as medical and commercial electronics.
The conformal coating production line, whether it is an operator manually brush coating printed circuit boards (PCBs) or an inline robotic spray coating process is typically made up of several stages.
However, not all the stages are mandatory or may be required. These stages are shown below:
Consider each stage below.
Boards In / Boards Out
This is very straightforward. The PCBs are delivered to the conformal coating area ready for processing.
Inspection
Inspecting the circuit boards before starting the conformal coating application process starts helps avoid stopping the line if there is a problem.
The initial inspection process can also identify which PCBs are to be coated, the material to be used and how they are to be coated.
This can be done automatically as part of the production process or completed by the operator.
Cleaning
Cleaning the circuit boards before application of conformal coating may or may not be required.
This is a decision that should be made earlier on in the design stage before production actually starts.
However, if you do decide to clean then consider these guidelines:
Ensure that the cleaning process does not add more contaminants to the PCB than it started with due to entrapment of cleaning fluids.
Make sure the PCBs are dry enough for the conformal coating process and the cleaning process not leave water entrapped under components that could impede the application.
Ensure the cleaning process actually removes residues that may cause defects like de-wetting to minimise finishing at the end of the process.
Again, cleaning is for the engineers to decide. The option of whether you need to clean your circuit boards for conformal coating is a whole topic on its own.
Masking
Masking may not be required on the circuit board. However, generally there are very few circuit board assemblies that can be completely coated and have no areas that must not be conformal coated.
Some components on PCBs generally need to avoid having conformal coating applied to them.
Again, engineers need to consider their options on whether to use conformal coating masking as soon as possible.
Pre-Coating Inspection
Generally, it is more efficient to double check the masking process is correct before conformal coating application rather than repair the PCB after the process goes wrong because the masking process was incorrect.
This check can be manual or automated but it is highly valuable.
Coating Application
The one stage that cannot be avoided is the conformal coating applied to the PCB.
The conformal coating inspection process can be done manually or automatically. This can depend on the volume of PCBs and the level of sophistication required.
Again, it is down to operator training and using the right equipment to ensure that this is possible.
Also, at this stage it is possible to measure process factors like coating conformal thickness to check that the criteria are met.
Other factors to consider
The set up of a conformal coating production line regardless of the application method has many similar characteristics.
General requirements
Any coating facility will need the basic requirements put in place that would be standard for any piece of electronic manufacturing process. These include ESD systems, facilities for the machines, the environmental requirements and the normal Health & Safety (HSE) considerations.
Health & Safety (HSE)
HSE tends to be more important for conformal coatings since in general the coatings themselves are hazardous, or the way they are applied makes them potentially harmful to operators
Environment
Conformal coatings are sensitive to the environment that they are processed. Cleanliness could be critical, as can temperature and humidity.
Summary
Setting up a conformal coating production facility can be a straightforward process as long as all factors are considered.
Get this right and many of the problems that could occur during production will be avoided.
What is UL and how does it relate to conformal coating?
UL stands for Underwriter Laboratories. They are a global safety certification body for consumer electronics.
When a conformal coating states that it has a UL qualification, it means that the material has been independently tested by UL in one of their laboratories and passed a particular standard.
UL carry out the testing. It is independent. There is no self-certification available.
UL qualified conformal coatings are used in all areas of electronics protection including aerospace, industrial controls, automotive and telecommunication sectors.
What UL standard tests are used with conformal coating?
For conformal coating materials there are two standard tests typically used.
These are UL94 and UL 746E.
UL 94 Standard for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances testing.
UL 94 is a plastic material flammability standard. It classifies plastics according to how they burn in various orientations and thicknesses.
UL 94 measures a conformal coating material’s ability to extinguish or to spread the flame once a test specimen has been ignited.
This ability is ranked as a classification as below.
Classifications
For conformal coatings the coupons are normally tested Horizontally (HB).
Tests are normally conducted on coupons of the minimum approved thickness with different types of laminate.
The conformal coating thickness range also is normally specified.
V-2 burning stops within 30 seconds on a vertical specimen; drips of flaming particles are allowed.
V-1: burning stops within 30 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.
V-0: burning stops within 10 seconds on a vertical specimen; drips of particles allowed as long as they are not inflamed.
Most conformal coatings aim to achieve V-0 status.
UL 746E Standard Polymeric Materials: Industrial Laminates, Filament Wound Tubing, Vulcanized Fiber and Materials Used in Printed-Wiring Boards
The UL 746 test measures the resistance of the conformal coating to electrical ignition sources.
The conformal coating material’s resistance to ignition and surface tracking characteristics is described in UL 746E.
Need to find out more?
Click UL standards and conformal coating for further information on whether you need UL qualified conformal coatings or contact us directly and we can help you.
Cleaning circuit boards before conformal coating is a huge topic by itself. This is because cleaning circuit boards can be challenging.
However, successful cleaning of electronic circuits can be achieved by a variety of techniques.
The main methods of cleaning can include:
• Aqueous wash
• Semi-aqueous wash
• Solvent & chemical wash
• Plasma cleaning
The key to success in cleaning circuit boards is similar to the success made with conformal coating. You need to match the cleaning process, the cleaning materials and the circuit board together.
If you do this then this will give you the best results for cleaning the circuit board assembly.
Why clean circuit boards before conformal coating?
The cleaning of a printed circuit board (PCB) before conformal coating application is normally done for two key reasons:
These are:
• Contamination removal
• Process improvement
They have different effects on the lifetime of the circuit board but can be equally important.
What types of contamination may be present on a circuit board?
Cleaning is used to remove many different types of contaminants from the manufacturing and assembly processes.
The residues can come from:
• Board laminate manufacture
• Component manufacture
• Soldering assembly processes (fluxes)
• Glue and ruggedizing processes
• Operator handling (finger prints, hair)
• Machine contamination (oils and greases)
• Environmental contamination (dust)
Removing the contamination may be a priority depending on their harmfulness.
Need to find out more?
Click conformal coating cleaning for further information or contact us directly and we can help you.
If you are new to Nexus and our work on conformal coatings then a good place to go is our Start Here page or our free conformal coating eBook.
Selective spraying of conformal coating using an automated robot system is one of the widest used application methods in high volume processing.
The principle is that the conformal coating is applied selectively by a small spray gun to the circuit board to the areas requiring coating only.
The selective process deliberately does not apply the conformal coating to areas that normally require masking such as connectors and other components.
This selective application of the conformal coating to the circuit board can avoid using a time consuming masking process and costs can normally be reduced.
What equipment do you need for selective spraying?
A specialist robotic system designed for application of conformal coatings is normally required for selective coating.
So, you are considering outsourcing your conformal coating or Parylene process to a subcontract supplier.
What’s the next step?
Assuming you have decided this is the right choice then the next task is to choose the right service provider.
This can be as difficult as selecting the right PCB manufacturer or laminate provider. There are good contractors and there are others. There are small providers and there are large-scale turnkey solutions.
The obvious answer is “the right one for me” but how do you ensure that you make the right decision?
Here are a few of the questions you should ask before signing up to the wrong coating solution provider and stop you wishing you had kept the coating process in house.
There are several questions you should ask before signing up to the wrong coating solution provider and stop you wishing you had kept the coating process in house!
The three key points to consider when choosing a subcontractor
When considering the supplier look at the three main areas.
The order you look at is up to you but ultimately you need to be happy in all three to keep sending the work out.
These three key areas are:
Quality
Turnaround time
Price
If you get these three key areas correct then you have succeeded. Everyone will be happy and there are no more problems with conformal coating.
However, within each of these areas, lie a lot of questions to be asked and if one of them isn’t working properly then who knows what the consequences are around the corner.
So, let’s consider each of these areas in turn in the order they are listed.
Quality
Everyone wants good quality. But what does that mean?
Whose quality are we judging against and how do we reach agreement?
One of the most important factors in subcontract conformal coating services is to agree what the PCB coating finish should look like.
Sound simple?
It can be if you define exactly what you want as a customer. The problem comes when you don’t know!
The reality is most customers have a preconceived idea of what a conformal coated printed circuit board should look like and the key for the customer is to communicate this to the provider.
Let me give you an example.
Consider a simple connector on a circuit board like the one below.
Then decide on what statement you agree with below:
Only the pins must not have coating on but the rest of the connector body does not matter.
The whole of the connector must have no coating on it all but there can be a gap of 1-2mm around it free of coating.
The whole of the connector must have no coating on it all and there can be no area around the connector free of coating.
All three options provide a connector free of coating. All three options work. All three options could be considered fine by various different customers.
However, the order they are stated is also lowest difficulty (aka cost) to highest difficulty to actually complete the work in a coating production line.
So, defining how you want to coat the board intimately impacts on the price of the project.
A key issue highlighted
Unfortunately, this highlights a key problem in conformal coating processing.
There are no standards that state what is the best solution and only guidelines. Ultimately, it comes down to a decision made by the customer as to how the PCB should be coated.
This means it is a crucial factor for both the customer and the supplier to define the level of quality. Get this right and most of your problems are sorted.
If the coating house is good they will help you define this from the beginning. They will not assume any level of quality but ask you what you need.
If they don’t help you then hope that they can guess what they need to provide you with.
A key problem in conformal coating processing is that there are no standards that state what is the best conformal coating finish and only guidelines. Ultimately, it comes down to a decision made by the customer as to how the PCB should be coated.
So what’s the next stage in quality?
So, you have agreed what quality of finish you want. Assuming that you know which conformal coating material you want then the rest should be easy.
Well, that statement is right as long as:
The supplier knows how to use your coating material correctly
Has the right equipment to apply the conformal coating
Has selected the right process for your PCB
Knows what to do when things go wrong
So, what you really need to do is find a subcontract conformal coating supplier that knows a lot about conformal coatings or Parylene.
Turnaround Time
So, you are considering outsourcing the coating work to an outside contractor but want the PCB coated when you need it.
Unfortunately, conformal coating is normally one of the last processes in a long line of operations so any delays in the manufacture of the PCB is normally being compounded by sending it out to a coating house.
Therefore, you need a fast turnaround option and your coating service should be flexible on this, allowing you choices on getting the PCBs coated.
However, you may want to consider the speed you require the PCB to be coated since it can lead to:
Extra costs
Potential problems with the process
Mistakes due to staff and machine availability
Availability of material
Local or global supply
Low cost offshore facilities
Capacity
These factors can influence the price significantly.
So, we have examined quality and turnaround time. The critical factor that ties these two areas together is price. Let’s take a look at this area.
Price
“I want this PCB coated for 45 cents”, says the customer.
“Okay can we see the board”, says the coating service provider.
The customer produces a 12”x6” PCB with 25 surface mount connectors with via’s everywhere and asks for coating both sides and wants all of the connectors not to be coated.
Okay, we have a mismatch in perception and this sounds ridiculous. But, it happens more regularly than it should.
Some customers have no idea of what it costs to coat a PCB. After all it’s just coating.
However, it’s up to the coating house to educate them so that they can get what they need and reach the happy point of all three areas satisfied.
So, what price should it be?
Well factors to be considered by the subcontractor are:
Material specified by the customer
Process to be used on the PCB
Volume of PCBs to be supplied
Amount of masking / keep out areas on the PCB
Amount of coating to be used per PCB
Is there cleaning involved before coating?
Yes cleaning is required, extra cost added in
No cleaning is not required, may be extra cost for finishing process if a lot of contamination
Speed of turnaround required (impact on resources, drying, curing)
Quality required (how much time finishing, inspecting, how close to look?)
So, the last two factors, turnaround and quality, tie directly back to the price and in reality have a huge impact.
Therefore, we find unsurprisingly all free factors should be considered as a whole and not separately.
Some customers have no idea of what it costs to conformal coat a circuit board. After all it’s just coating….
Summary
Subcontracting out your conformal coating process isn’t difficult if you look carefully at the three areas of quality, turnaround time and price.
If you can achieve all three with your subcontract coating house then you will be happy.
If you ignore one of the factors then it may be a less pleasant position.
Parylene is a conformal coating that can be applied to electronic circuit board assemblies that is deposited as a gas in a vacuum chamber.
